Superalloys of nickel, cobalt or nickel-iron base alloying element are often used in extreme heat and corrosive environments such as the turbine blades and vanes of a gas turbine engine. To protect the superalloy components from the heat, oxidation and corrosion effects of the impinging hot gas stream, the superalloy components are protected by thermal barrier coating (TBC) systems. A typical TBC system has a three-layer structure where an outer coat of ceramic layer provides the thermal protection. The ceramic layer is typically a yttria-stabilized zirconia (YSZ). A thin metallic layer or bond coat layer is applied under the ceramic layer to provide adhesion between the ceramic layer and the superalloy substrate. The metallic bond coat layer is generally aluminum based alloy such as nickel aluminide, cobalt aluminide or platinum aluminide. Subsequently, a layer of aluminum oxide scale is thermally grown at the interface between the metallic bond coat layer and the ceramic layer. The metallic bond coat layer serves as an aluminum reservoir for the formation of the adherent aluminum oxide scale layer. This thermally grown aluminum oxide scale protects the superalloy substrate from oxidative corrosion. Oxygen readily diffuses through the YSZ ceramic layer and the aluminum oxide resists the oxidizing effects of the hot combustion gas stream.
Unfortunately, the coefficient of thermal expansion (CTE) of alumina is considerably lower than that of the underlying superalloy metal substrate. Upon thermal cycling of the superalloy components, the CTE mismatch causes stress to accumulate in the growing oxide layer. Once the thickness of the oxide reaches a critical value (around 10 μm), the stresses become so large that they must be alleviated by either creep or plastic deformation, which leads to spalling of the coating layers and failure of the TBC system. Thus, slowing the oxide growth and/or increasing its creep resistance are ways to extend the life of the TBC system.